This invention relates generally to a hydraulic fluid power circuit, and more particularly to a motor vehicle hydraulic fluid power circuit having a hydraulic pump for supplying fluid to assist the operator in steering and braking the vehicle.
Motor vehicles are frequently provided with a hydraulic pump which is driven by the engine of the vehicle to provide a source of fluid power. The hydraulic pump may be a constant displacement pump which provides a constant output for any shaft speed, and the output of the pump may be hydraulically connected to a control valve which selectively directs the pump output to a steering valve to assist in steering the vehicle or to a brake booster accumulator to assist in braking. The control valve normally directs the pump output to the steering valve when the accumulator is fully charged and directs the pump output to the accumulator when the accumulator pressure is depleted below a minimum charge pressure. Additionally, the control valve may also interrupt charging of the accumulator and direct the pump output to the steering valve if the steering valve is actuated while the accumulator is being charged, so that the entire pump output is available to the steering valve at all times.
The steering valve of the circuit is an open center steering valve which directs the fluid which it receives from the pump to a drain line when the steering valve is in a neutral or centered position, and which directs at least a portion of such fluid to a power steering fluid motor when the valve is moved from its center position. The brake booster of the circuit is a closed center device which utilizes the fluid from the accumulator to apply a force to the brake system master cylinder in response to an input force on the brake pedal of the vehicle.
The brake booster accumulator provides reserve capacity for the brake booster in the event of failure of the fluid power supply, and minimizes the output requirements of the pump by permitting the pump to be used for charging the accumulator when the pump output is not otherwise required for steering purposes. The circuit permits standardization of components, since the circuit utilizes a constant displacement hydraulic pump and an open center steering valve of the type which are used in circuits that do not include an accumulator or a hydraulically actuated brake booster.